U.S. Sen. Al Franken and six Democratic colleagues in the Senate penned letters to former Internal Revenue Service Commissioner Douglas Shulman in February 2012, asking whether the IRS intended to investigate social welfare organizations "engaged in a substantial or even a predominant amount of campaign activity."

"We urge you to protect the legitimate section 501(c)(4) entities by preventing non-conforming organizations that are focused on federal election activities from abusing the tax code," the Feburary letter read, in part.

The letter sent to Shulman in March had a more stern tone and recommendations for cracking down on "questionable practices."

"We urge the IRS to take ... steps immediately to prevent abuse of the tax code by political groups focused on federal election activities. But if the IRS is unable to issue administrative guidance in this area then we plan to introduce legislation to accomplish these important changes," the March letter read, in part.

In the wake of the IRS admission that staff targeted dozens of tea party and conservative groups for extra scrutiny during the tax-exempt status application process, Republicans in Washington, D.C., and Minnesota have tried to pin blame on the Democratic senators.

On Thursday, Republican Minnesota Sen. Julianne Ortman criticized Franken, saying the letter contributed to a culture in Washington that allowed the targeting.

During an appearance on CNN on Monday, Sen. Franken called for a "non-partisan" inquiry into all 501(c)(4) groups.

Last year's letters -- signed by Franken, Chuck Schumer of New York and several others -- do not request that the IRS specifically target tea party or conservative groups. It was also sent more than a year after the IRS began targeting the groups.